1. Field of the Invention
This invention relates to measuring mechanical stress in thin films. More specifically, the invention is a system for measuring mechanical stress in a thin film during thin-film fabrication that includes film deposition and thermal annealing processes.
2. Description of the Related Art
Control of residual stress in substrate-supported thin films has historically presented challenges in the fabrication of thin-film sensors, devices, and optics utilizing substrate-supported thin films. The stress can cause buckling or cracking of the film, delamination from the substrate, and/or substrate deformation. Measurement and control of thin-film stress is necessary for the successful fabrication of thin-film-based devices (e.g., semiconductors, optics, etc.) used in the fields of chemistry, mechanics, magnetics, and electricity.
Stress-induced deformation is of particular concern in the fabrication of reflective focusing and collimating X-ray optics used is a broad range of application-specific devices to include medical imagers and space telescopes. Briefly, the stress experienced by a device's thin-film elements can alter the precise geometrical figure of a thin-film supporting substrate thereby degrading the device's focusing or collimating properties. A thin-film reflective coating can be composed of a single-layer metal film, or hundreds of alternating angstrom-scale multilayered material pairs (known as X-ray multilayers in the art). For X-ray optics that utilize multilayer coatings, the negative impact to the overall optical performance resulting from thin-film stress-induced figure errors is two-fold in comparison to single-layer films. Briefly, multilayer coatings are designed so that a maximum in reflectivity occurs at a precise grazing incidence angle for a given substrate geometry and photon energy according to the Bragg condition. Therefore, stress-induced substrate deformation can cause an effective change in the optic's designed grazing incidence angle that, in turn, can cause a significant reduction in the focusing or collimating properties and the X-ray reflectivity.
For most thin-film deposition techniques, the stress experienced by the thin film and substrate is highly process dependent and can be minimized to various degrees depending on material composition through the optimization of the many process parameters. Since thin-film and/or substrate stresses can be reduced or eliminated through adjustment of thin-film fabrication process, it is desirable to monitor stress in a thin-film and its supporting substrate during the fabrication process. Current approaches to in-situ stress monitoring involve the use of laser-based optical systems that direct one or more laser beams towards the surface of the thin film being deposited on a substrate and measure beam deflection using precision-placed cameras/sensors. The measured relative displacement of the deflected beam is geometrically related to the stress-induced substrate curvature from which the stress can be calculated using the well-known Stoney equation. Unfortunately, there are two major drawbacks with these approaches. First, difficulties can be encountered for deflectometry measurement techniques when attempting to measure the stress in transparent films. Since the light is incident on the film side, destructive interference between the film and substrate can occur when the optical thickness of the film approaches a quarter of a wavelength of the incident laser light. If the reflected light received by the detectors is not of sufficient intensity, detectors can fail to track the signal and stall. Second, to avoid contamination of a vacuum deposition chamber required for thin-film fabrication, laser-based monitoring equipment is mounted outside of the deposition chamber and focused into the deposition chamber via optical windows built into the chamber walls. This leads to cost/size/complexity issues for in-situ stress monitoring.